The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with
pp
collisions at a centre-of-mass energy
s
=
13
TeV. The absolute luminosity scale is determined using ...van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosity for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2
pp
data sample corresponds to an integrated luminosity of
140.1
±
1.2
fb
-
1
, i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017–2018 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of
338.1
±
3.1
pb
-
1
.
Celotno besedilo
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search is presented for four-top-quark production using an integrated luminosity of 139 fb
-
1
of proton–proton collision data at a centre-of-mass energy of
13
TeV
collected by the ATLAS detector ...at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be
24
-
6
+
7
fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb(-1) of pp collision data at a centre-of-mass ...energy of root s =13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.
A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic
t
t
¯
plus missing transverse momentum final ...state is presented. The analysis of 139
fb
-
1
of
s
=
13
TeV proton–proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via
t
~
→
t
(
∗
)
χ
~
1
0
, with
t
(
∗
)
denoting an on-shell (off-shell) top quark and
χ
~
1
0
the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where
m
t
~
>
m
t
+
m
χ
~
1
0
, top squark masses are excluded in the range 400–1250 GeV for
χ
~
1
0
masses below 200 GeV at 95% confidence level. In the situation where
m
t
~
∼
m
t
+
m
χ
~
1
0
, top squark masses in the range 300–630 GeV are excluded, while in the case where
m
t
~
<
m
W
+
m
b
+
m
χ
~
1
0
(with
m
t
~
-
m
χ
~
1
0
≥
5
GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300–660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A technique is presented to measure the efficiency with which
c
-jets are mistagged as
b
-jets (mistagging efficiency) using
t
t
¯
events, where one of the
W
bosons decays into an electron or muon ...and a neutrino and the other decays into a quark–antiquark pair. The measurement utilises the relatively large and known
W
→
c
s
branching ratio, which allows a measurement to be made in an inclusive
c
-jet sample. The data sample used was collected by the ATLAS detector at
s
=
13
TeV
and corresponds to an integrated luminosity of 139 fb
-
1
. Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and
t
t
¯
decay products that yields the highest likelihood value. The distribution of the
b
-tagging discriminant for jets from the hadronic
W
decays in data is compared with that in simulation to extract the mistagging efficiency as a function of jet transverse momentum. The total uncertainties are in the range 3–17%. The measurements generally agree with those in simulation but there are some differences in the region corresponding to the most stringent
b
-jet tagging requirement.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The inclusive top quark pair (
t
t
¯
) production cross-section
σ
t
t
¯
has been measured in proton–proton collisions at
s
=
13
TeV
, using 36.1 fb
-
1
of data collected in 2015–2016 by the ATLAS ...experiment at the LHC. Using events with an opposite-charge
e
μ
pair and
b
-tagged jets, the cross-section is measured to be:
σ
t
t
¯
=
826.4
±
3.6
(
stat
)
±
11.5
(
syst
)
±
15.7
(
lumi
)
±
1.9
(
beam
)
pb
,
where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on
m
t
pole
, giving
m
t
pole
=
173
.
1
-
2.1
+
2.0
GeV
. It is also combined with measurements at
s
=
7
TeV
and
s
=
8
TeV
to derive ratios and double ratios of
t
t
¯
and
Z
cross-sections at different energies. The same event sample is used to measure absolute and normalised differential cross-sections as functions of single-lepton and dilepton kinematic variables, and the results are compared with predictions from various Monte Carlo event generators.
Celotno besedilo
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search for new phenomena in final states with hadronically decaying tau leptons, b-jets, and missing transverse momentum is presented. The analyzed dataset comprises pp collision data at a ...center-of-mass energy of root s = 13 TeV with an integrated luminosity of 139 fb(-1), delivered by the Large Hadron Collider and recorded with the ATLAS detector from 2015 to 2018. The observed data are compatible with the expected Standard Model background. The results are interpreted in simplified models for two different scenarios. The first model is based on supersymmetry and considers pair production of top squarks, each of which decays into a b-quark, a neutrino and a tau slepton. Each tau slepton in turn decays into a tau lepton and a nearly massless gravitino. Within this model, top-squark masses up to 1.4 TeV can be excluded at the 95% confidence level over a wide range of tau-slepton masses. The second model considers pair production of leptoquarks with decays into third-generation leptons and quarks. Depending on the branching fraction into charged leptons, leptoquarks with masses up to around 1.25 TeV can be excluded at the 95% confidence level for the case of scalar leptoquarks and up to 1.8 TeV (1.5 TeV) for vector leptoquarks in a Yang-Mills (minimal-coupling) scenario. In addition, model-independent upper limits are set on the cross section of processes beyond the Standard Model.
A
bstract
A search for Higgs boson pair production via vector-boson fusion (VBF) in the
b
b
¯
b
b
¯
final state is carried out with the ATLAS experiment using 126 fb
−
1
of proton- proton collision ...data delivered at
s
= 13 TeV by the Large Hadron Collider. This search is sensitive to VBF production of additional heavy bosons that may decay into Higgs boson pairs, and in a non-resonant topology it can constrain the quartic coupling between the Higgs bosons and vector bosons. No significant excess relative to the Standard Model expectation is observed, and limits on the production cross-section are set at the 95% confidence level for a heavy scalar resonance in the context of an extended Higgs sector, and for non-resonant Higgs boson pair production. Interpretation in terms of the coupling between a Higgs boson pair and two vector bosons is also provided: coupling values normalised to the Standard Model expectation of
κ
2
V
< −
0
.
76 and
κ
2
V
>
2
.
90 are excluded at the 95% confidence level in data.
This paper presents an analysis at next-to-next-to-leading order in the theory of quantum chromodynamics for the determination of a new set of proton parton distribution functions using diverse ...measurements in
pp
collisions at
s
=
7
, 8 and 13 TeV, performed by the ATLAS experiment at the Large Hadron Collider, together with deep inelastic scattering data from
ep
collisions at the HERA collider. The ATLAS data sets considered are differential cross-section measurements of inclusive
W
±
and
Z
/
γ
∗
boson production,
W
±
and
Z
boson production in association with jets,
t
t
¯
production, inclusive jet production and direct photon production. In the analysis, particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. The resulting set of parton distribution functions is called ATLASpdf21.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
The results of a search for new phenomena in final states with
b
-jets and missing transverse momentum using 139 fb
−
1
of proton-proton data collected at a centre-of-mass energy
s
= 13 TeV ...by the ATLAS detector at the LHC are reported. The analysis targets final states produced by the decay of a pair-produced supersymmetric bottom squark into a bottom quark and a stable neutralino. The analysis also seeks evidence for models of pair production of dark matter particles produced through the decay of a generic scalar or pseudoscalar mediator state in association with a pair of bottom quarks, and models of pair production of scalar third-generation down-type leptoquarks. No significant excess of events over the Standard Model background expectation is observed in any of the signal regions considered by the analysis. Bottom squark masses below 1270 GeV are excluded at 95% confidence level if the neutralino is massless. In the case of nearly mass-degenerate bottom squarks and neutralinos, the use of dedicated secondary-vertex identification techniques permits the exclusion of bottom squarks with masses up to 660 GeV for mass splittings between the squark and the neutralino of 10 GeV. These limits extend substantially beyond the regions of parameter space excluded by similar ATLAS searches performed previously.
PDF
